1. Field of the Invention
This invention relates to the art of papermaking, particularly to treating of formed paper product with heat and subsequent rewetting to improve its properties, including dry and wet stiffness, wet tensile strength and opacity.
2. Description of the Prior Art
In the art of papermaking, it is customary to subject felted fibers to wet pressing and then to drying on heated rolls.
There is currently considerable interest in improving various properties of paper and boards. Quantifiable paper properties include: dry and wet tensile strength, folding endurance, stiffness, compressive strength, and opacity, among others. Which qualities should desirably be enhanced depends upon the intended application of the product. In the case of milk carton board, for example, stiffness is of utmost importance whereas for linerboard, three qualities of particular interest to us are wet strength, folding endurance, and high humidity compression strength.
All of these properties can be measured by well-known standard tests. As used herein, then, "wet strength" means wet tensile strength as measured by American Society for Testing and Materials (ASTM) Standard D829-48. "Folding endurance" is defined as the number of times a board can be folded in two directions without breaking, under conditions specified in Standard D2176-69. "Stiffness" is defined as flexural rigidity and is determined by the bending moment in g-cm. "Linerboard", as used herein, is a medium-weight paper product used as the facing material in corrugated carton construction. Kraft linerboard is linerboard made according to the kraft process, and is well known in the industry. Folding carton board is a medium to heavy weight paper product made of unbleached and/or bleached pulps of basis weights from 40-350 g/m.sup.2.
Prior workers in this field have recognized that high-temperature treatment of linerboard can improve its wet strength. See, for example E. Back, "Wet stiffness by heat treatment of the running web", Pulp & Paper Canada, vol. 77, No. 12, pp. 97-106 (December 1976). This increase has been attributed to the development and cross-linking of naturally occurring lignins and other polymers, which phenomenon may be sufficient to preserve product wet strength even where conventional synthetic resins or other binders are entirely omitted.
It is noteworthy that wet strength improvement by heat curing has previously been thought attainable only at the price of increased brittleness (i.e., reduced folding endurance). Embrittled board is not acceptable for many applications involving subsequent deformation, and therefore heat treatment alone, to develop the wet strength of linerboard and carton board, has not gained widespread acceptance. As Dr. Back has pointed out in the article cited above, "the heat treatment conditions must be selected to balance the desirable increase in wet stiffness against the simultaneous embrittlement in dry climates." Significantly, in U.S. Pat. No. 3,875,680, Dr. Back has disclosed a process for heat treating already manufactured corrugated board to set previously placed resins, wherein the specific purpose is to avoid running embrittled material through a corrugator.
It is plain that improved stiffness and wet strength, on one hand, and improved folding endurance, on the other, where previously thought to be incompatible results.
It is therefore an object of the invention to produce paper product having both improved stiffness and wet strength, and improved folding endurance. Another goal is to achieve that objective without resorting to synthetic resins or other added binders.
With a view to the foregoing, a heat treatment process has been developed which dramatically and unexpectedly increases not only the stiffness and wet strength of different boards, but also preserves their folding endurance. In its broadest sense, the invention comprises steps of (1) heating a board produced from either unbleached or bleached kraft pulp to an internal temperature of at least 400.degree. F. (205.degree. C.) for a period of time sufficient to increase the wet strength of the board; and (2) rewetting the board immediately after the heat treatment to at least 1% moisture by weight. These steps are followed by conventional drying and/or conditioning of the treated board. It is to be understood that steps 1 and 2 can be repeated several times.
This method produces a product having folding endurance greatly exceeding that of similar board whose stiffness and wet strength have been increased by heat alone. This is clearly shown by our tests exemplified below.
Of course, those skilled in the art will recognize the necessity of the product conditioning to a normal moisture content after this very hot treatment. See, for example, U.S. Pat. No. 3,395,219. A certain amount of rewetting is normally done, and in fact product properties are never even tested prior to conditioning. All conventional rehumidification is done after the product has substantially cooled.
Our treatment principally differs from conditioning in that we add water, by spraying or otherwise, to a very hot and dry paper or board at the very end of the heat treatment, without intermediate cooling. It is critical to our process that water be applied to the product while it is still hot, certainly above 100.degree. C. (212.degree. F.), and preferably above 205.degree. C. (400.degree. F.). Another heat treatment or drying step may follow rewetting, on or off the machine, during a subsequent operation such as sizing, coating or calendering.
While the invention may be practiced over a range of temperatures, pressures and duration, these factors are interrelated. For example, the use of higher temperatures requires a heat treating step of shorter duration, and vice-versa. For example, at 550.degree. F. (289.degree. C.), a duration of 2 seconds has been found sufficient to obtain the desired improvements, while at 420.degree. F., considerably longer is required.
We prefer to raise the internal temperature of the board to at least 450.degree. F. (232.degree. C.) during the heat treating step, as greater stiffness and wet strength are then achieved. This may be because at high temperatures, shorter step duration is necessary to develop bonding, and there is consequently less time for fiber degradation to occur. Also, shorter durations enable one to achieve high production speeds.